1. Field of Invention
This invention relates to the field of the radio telephone apparatus such as cellular radio telephones and cordless telephones and, more particularly, to an improved portable radio telephone apparatus couplable to an adapter unit.
2. Description Of The Relevant Art
As cellular radio telephones come into wide use, various types of vehicle adapters for the radio telephone have been developed. For example, Aisaka et al. U.S. Pat. No. 5,048,117 discloses an adaptor unit which may amplify radio frequency signals output from a portable radio telephone in response to a level control signal. As another example of the vehicle adaptor, commonly assigned Siawa et al. U.S. Pat. No. 5,045,115 discloses a radio telephone apparatus wherein a portable radio device may be connected to an additional unit having a speaker and a microphone via a cable, so that hand-free operations becomes available to the user of the apparatus when they are connected.
According to such conventional adapters, however, two power supply lines are required to be included in a connection cable between the portable radio telephone and the adaptor unit: one for charging the battery of the portable radio telephone and the other one for supplying the portable radio telephone with electric power. When this power supply-using two lines is used, the circuit arrangement of the portable radio telephone becomes so complicated that the cost in manufacturing the radio telephone increases.